In general, in digital recording, information is encoded in the spacing of magnetic polarity reversals. During the reading process, the polarity reversals are converted into voltage pulses, so that the information in the read signal is encoded in the spacing of pulse peaks. Therefore, for data integrity, the accuracy of the timing of the peaks in the read signal is critical. There are, however, inherent distortions of the read signal that must be accommodated.
Binary information on magnetic disks and tapes is stored by magnetizing small areas of the magnetic surface with one of two polarities. For flexible disks and tapes, the small magnetized areas are typically recorded primarily longitudinally (in the plane of the surface). Although the magnetized areas are primarily longitudinal, they also have an unavoidable partial vertical component (perpendicular to the surface). During reading, a magnetic coil in a read head intersects magnetic fields from the small magnetized areas. As the head passes through the fields, a transition from one polarity to the opposite polarity results in a changing field that in turn induces a changing current in the coil of the read head. The changing current typically drives a resistive load to provide a voltage signal. Ideally, for purely longitudinal magnetization, an isolated transition from one polarity to the opposite polarity results in a perfectly symmetrical voltage pulse. Ideally, for purely vertical magnetization, an isolated transition from one polarity to the opposite polarity results in a perfectly symmetrical dipulse (a pulse of one polarity followed by a pulse of the opposite polarity). The combined longitudinal and vertical components result in an asymmetrical voltage pulse. The asymmetry is sometimes referred to as a peak shift. As bit densities increase, the transitions are no longer isolated. In particular, if two adjacent write transitions are very close together, the combined effects of peak shift and adjacent pulses make the time between read pulses on read-back longer than the time between the transitions during writing.
Given a particular digital pattern in the write waveform, it is possible to predict some of the resulting distortion in the read waveform. It is then possible to distort the writing signal (called precompensation or write equalization) to make the resulting read signal closer to ideal. For example, if two adjacent write transitions are very close together, then the first transition may be written late and the second transition written early. As a result, the time between transitions during writing is shorter than the ideal time and the time between pulses during read back is equal or closer to the ideal time. Alternatively, the shape of the analog read signal can be affected by inserting extra pulses in the write waveform. Typically, the frequency response of the magnetic head is such that these extra pulses do not result in complete polarity reversals in the surface of the data storage medium, but instead the fields on the magnetic medium are slightly modified (write equalized) to compensate for distortion and noise in the resulting read signals. For additional general background, see R. J. Schneider, "Write equalization in high-linear-density magnetic recording," IBM J. Res. Develop., vol. 29, No. 6, 563-568 (November 1985).
Many removable-medium drives are adapted to accept a variety of magnetic media. Typically, different media types require different write equalization parameters. For example, flexible disk drives may accept flexible disks having two different recording densities. Likewise, tape drives may accept multiple types of tape. In some cases, after a particular drive has been shipped, a new magnetic medium may become commercially available having a new higher bit density or a different encoding pattern and therefore requiring different specifications for write compensation. In that case, it would be useful to be able to program the write equalization circuitry to accommodate the new media. There is an ongoing need for write equalization circuitry that can be programmed to accommodate a variety of equalization specifications.